In recent years, soft silicone hydrogel contact lenses, for example, Focus NIGHT & DAY™ and O2OPTIX™ (CIBA VISION), and PureVision™ (Bausch & Lomb) become more and more popular because of corneal health benefits provided by their high oxygen permeability and comfort. Currently available silicone Hydrogels contact lenses are typically produced according to full molding processes involving disposable molds and a polymerizable mixture including at least one hydrophilic monomer, at least one silicone-containing monomer or macromer, and a solvent. However, no made-to-order (MTO) or customized silicone hydrogel contact lenses are commercially available.
MTO or customized contact lenses can match a patient's prescription and/or have a base curve desired by the patient. But, there are at least two problems associated MTO or customized contact lenses. First, a silicone hydrogel material is generally soft and sticky. It can only be lathed at low temperature and low temperature lathing can have a relatively high operation cost. Commonly-assigned copending U.S. patent application Ser. No. 11/148,104 disclosed methods for making room-temperature lathable silicone hydrogel materials.
The second problem is the difficulty to consistently produce a room-temperature lathable silicone hydrogel material with desired physical and mechanical properties (e.g., oxygen permeability, ion permeability, elastic modulus (modulus), elongation strength, etc.). Typically, a polymer rod is first produced and then lathed to produce a MTO or customized contact lens. A polymer rod is obtained by slowly polymerizing a lens-forming material in long glass tubes under well controlled conditions (e.g., temperatures). This type of process has a number of disadvantages that are inherent to bulk polymerization. First, the polymerization is highly exothermic and reaction kinetics are highly temperature dependent. Dissipation of heat and therefore maintaining a uniform temperature in the polymerization is a challenge since there is no stirring or solvent present during the polymerization process. Control of temperature within polymerization tubes is further complicated by increases in viscosity as conversion of monomer increases. These effects can lead to uncontrolled polymerization (run away reaction) and large temperature gradients within the polymerization tube. A polymer produced during uncontrolled polymerization can have non-homogenous composition, variability in the sequence of monomers and/or macromers in the polymer chain, and/or physical defects such as cracks and voids, resulting in low yield and high production cost. As such, polymerization of a lens-forming material in a tube generally is performed very slowly at a relatively low temperature, e.g., at 45° C. A slight increase in temperature, for example, an increase of about ten degrees, would result in formation of inhomogeneous material and defects such as cracks and voids and thereby decrease dramatically the yield of polymer rods suitable for making MTO contact lenses. Furthermore, variability in the sequence of monomers and/or macromers in the polymer chain or composition can result in variation in properties such as oxygen permeability (Dk), ion permeability (IP) and mechanical strength.
Therefore, there are needs for methods for consistently preparing a silicone hydrogel material with desired properties.